Apparatus including a flow path formed by membrane compression

ABSTRACT

Apparatus for altering the concentration of a pre-selected component (11) of a feestock (12) wherein the feedstock (12) flows through flowpaths bounded by overlying barriers (13) which selectively pass the component (11). Flow of feedstock (12) elastically separates the barriers (13) to maintain laminar flow therebetween. The preselected component (11) may be routed to flow selectively through the barriers (13) either into or out of the feedstock (12).

This application is a continuation of U.S. application Ser. No. 375,139,filed Apr. 28, 1982, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to apparatus for altering theconcentration of a pre-selected component of a feedstock. The inventionis applicable to the removal of micron and submicron species from afluid phase but it is to be understood that it is not limited thereto

For the sake of convenience, the invention will be described in relationto cross-flow retention or filtration in which the pre-selectedcomponent or specie is removed from the feedstock by transfer through abarrier adapted to pass the component or specie and to retain theremainder of the feedstock. However, it is to be understood that theinvention is not limited thereto as it may equally be applied to thereverse situation in which the preselected specie is introduced throughthe barrier to the feedstock.

Micron and submicron species (for example, molecules, colloids,particles and droplets) in a fluid phase (for example, an aqueous phase)can be removed in a number of different ways depending on the quantityof species present.

For low concentrations, depth filtration is probably the most commonmethod applied. An alternative approach is to use a surface filter, forexample the Nucleopore membrane; this membrane effects removal ofparticulates by a surface sieving mechanism. Other types of surfacemembranes are available; these rely on an active surface skin which isbacked up by a porous support layer. In particulate filtrationoperations such membranes behave similarly to Nucleopore membranes.

When the concentration of retained species is high, depth and dead-endsurface barriers become much less attractive, as the pressure dropnecessary to effect filtration increases rapidly with solidsaccumulation.

It is to overcome this problem that a new area of micron and submicronspecies retention is being developed. The technique used is cross-flowretention. In cross-flow retention a surface membrane is used and thebuild-up of a layer of retained species is minimised by applying a fluidshear field to the upstream fluid adjacent to the barrier surface. Thiscan be done either by stirring or by pumping the fluid across thebarrier surface.

According to this technique, it is quite feasible to operate at a steadystate in which a solution is effectively split into a permeate and aretentate, and at steady state no more species accumulates at thebarrier surface, causing it to lose performance. Alternatively, thesystem can be operated in a batch mode, and in this case the feedsolution gradually increases in concentration, and although this leadsto a drop in throughput, the drop is far less than would occur in thedead-end mode where all of the species collect on or in the barrier.

Generally, for each membrane application in the fields ofultrafiltration, dialysis and electrodialysis, the permeability of themembrane system is generally limited by the layer of retained species(i.e. the concentration layer or, eventually the gel layer) which ispresent. According to the present invention, it is preferred thatlaminar flow be employed to remove the gel or cake from the surface ofthe membrane.

In the case of laminar flow there is a relationship for a givenconcentration of a fluid to be treated and for a given membrane. Thisrelationship links flux, shear rate and length of the filter flowpath.

As given by Blatt et al in 1970, in Membrane Science and Technology, therelationship is in respect of laminar flow conditions and for the gelpolarized condition (i.e. when increased pressure does not increaseflux). The relationship is given as:

    Jα(γ/L).sup.0.33 α(U.sub.B).sup.0.33 (h.sub.c).sup.-0.33 (L).sup.-0.33

wherein

J is the flux for a given area of membrane

U_(B) is the velocity of the fluid

h_(c) is the height or thickness of the filter flowpath

L is the length of the filter flow path

γ is the shear rate

The shear rate is an expression of the ratio of ν the tangentialvelocity of fluid between adjacent membranes and the height of thefilter flowpath or channel, that is:

    γα(ν/hc)

When the gel layer (rather than the porosity of the membrane) is thelimiting factor to membrane performance, the flux is linked to the shearrate through this ratio. This means that the effect of reducing thechannel height or thickness is to increase significantly both the shearrate and the flux.

Generally, in cross-flow retention, the energy involved forrecirculation of the feed is the highest direct cost factor of theoperation. For classical systems, with h_(c) of the order of 1 mm,energy consumption is of the order of 1 kW/square meter of membraneinstalled. In the case of tubular systems, with tubes of diameter of theorder of 1 cm., the cost of energy required is even greater.

Accordingly, there is an interest in developing cross-flow capillaryretention or filtration and ultrafiltration apparatus wherein the heightof the flowpath is significantly reduced, for example to about 50-100microns. By using a flowpath height of only this magnitude, the pumpingcapacity required per square meter of membrane is proportionallyreduced; double the channel height and the pumping capacity required ismore than doubled.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention there is providedapparatus for altering the concentration of a pre-selected component(s)of a feedstock including:

(a) inlet means adapted to admit pressurized feedstock into theapparatus,

(b) outlet means adapted to remove treated feedstock from the apparatus,

(c) one or more feedstock flowpaths between the inlet means and outletmeans,

(d) one or more barriers adapted to pass said pre-selected component(s)and having a first surface past which the feedstock is directed, and,

(e) transfer means adapted to communicate with the opposite surface ofthe or each barrier,

and wherein the boundaries of the or each flowpath are at leastpartially defined by said one or more barriers and is adapted to be atleast partially elastically enlarged by the passage of feedstocktherethrough, and said apparatus further includes limiting means adaptedto restrict the extent of elastic enlargement of said one or moreflowpaths so as to maintain a laminar flow of said feedstook thereinwhen feedstock is flowing through said one or more flow paths at apredetermined operating pressure. Preferably, the flowpath thickness issuch that the apparatus operates under pre-gel polarised conditions sothat increased pressure does not increase flux. Although it is possiblefor the apparatus of the present invention to utilise some componentsdeveloped for prior art dialysis technology as referenced herein, itwill be appreciated that the cross-flow filtration and ultrafiltrationtechnology described herein differs quite significantly to the prior artdialysis technology and that the prior art dialysis apparatus is notsuitable as such for the high-pressure cross-flowfiltration/ultrafiltration application described herein. For example,the following differences may be noted:

(i) Dialysis is a four vector system for two fluids with apparatuscomprising two inlets and two outlets--an inlet and an outlet for thematerial to be dialysed and a separate inlet and outlet for thedialysing liquid which flows as a counter current to the material to bedialysed. Cross flow filtration, on the other hand, is a three vectorsystem for one fluid--therebeing with only one inlet for the feedstockto be treated and two separate outlets one for the concentrate orretentate and one for the filtrate or permeate

(ii) Dialysis results in a dilution of the material being dialysedwhereas cross-flow filtration results in concentration of the retainedspecies in the material being treated.

(iii) Dialysis operates at pressures of less than 10 KPa whereascross-flow filtration is performed at pressures of the order of 100 KPa.

(iv) Dialysis uses low water flux membranes at low flow rates (e.g. 2liters/day) with a minimum transmembrane pressure gradient. Cross-flowfiltration uses high water flux, highly permeable membranes (flow ratese.g. 50 liters/hour) with a large transmembrane pressure gradient.

(v) Dialysis uses a pair of membranes each of about 40 microns thicknesswith a channel height or thickness of about 150 microns at normalatmospheric pressure. Comparable cross-flow filtration apparatus uses apair of membranes each of about 120-200 microns thickness with a zerochannel height (i.e. the membranes are in contact) at normal atmosphericpressure, and a channel height or thickness of about 50 microns under anoperative transmembrane pressure gradient of 100 KPa.

As used in this specification, the term "barrier" includes high fluxsemi-permeable membranes, biofilters and filters which are bothcompressible and resilient and/or can be mounted on a compressible andresilient backing or support.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more readily understood and put intopractical effect, reference will now be made to the accompanyingdrawings in which:

FIG.1 is a perspective view (partly broken away) of a fluid treatmentapparatus or filter according to a first embodiment of the invention,

FIG. 2 is a side elevational view of the filter shown in FIG. 1,

FIG. 3 is a plan view of a backing plate of the filter shown in FIG. 1,

FIG. 4 is an exploded view of a filter unit of the filter shown in FIG.1,

FIG. 5 is a diagrmmatic view of a cross flow filter apparatusconstructed in accordance with the principles of the present inventionwith the filter in a pre-use condition;

FIG. 6 is a schematic view similar to FIG. 7 with the filter in thesteady state of feedstock flow,

FIG. 7 is an enlarged fragmentry cross-sectional view of a pair ofspacer support plates with a pair of compressible membranes disposedtherebetween.

FIG. 8 is a partial cross-sectional schematic view of a filter unitshowing the gasket seal at the edge of the two membranes with the filterin a pre-use condition,

FIG. 9 is a view similar to FIG. 8 with the filter in the steady stateof feedstock flow,

FIG. 10 is a schematic view showing a blockage forming in the flowpath,

FIG. 11 is a schematic view similar to FIG. 10 showing movement of theblockage,

FIG. 12 is a cross-sectional side view of a cross-flow filter accordingto a second embodiment of the invention,

FIG. 13 a partially cutaway perspective view of the filter shown in FIG.12,

FIG. 14 is a cross-sectional view of a membrane envelope of the filtershown in FIGS. 12 and 13,

FIG. 15 is a cross-sectional schematic view of a further embodiment ofthe invention, and,

FIG. 16 is a cross-sectional schematic view of yet another embodiment ofthe invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As shown in FIGS. 1 and 2, the preferred apparatus 10 for altering theconcentration of a pre-selected component(s) 11 of a feedstock 12comprises a plurality of barriers 13 adapted to pass said component(s)11. Inlet and manifold means 17 at the right handside of FIG. 1 (seeFIG. 2) are adapted to direct the feedstock into contact with a firstsurface of each barrier 13. Transfer means 14 are adapted to communicatewith the opposite surface of each filter membrane 13 to receive thepassed component(s) 11, for removal of said passed component(s) 11 fromthe apparatus 10. Outlet and manifold means 15 are adapted to remove thetreated feedstock 12 from the apparatus 10.

The boundaries of the flowpaths for the feedstock 12 which, in thisinstance, are as defined by the barriers 13 are adapted to be at leastpartially enlarged elastically by the passage of feedstock therethrough.

Limiting means in the form of plates 16 are adapted to restrict theextent of elastic enlargement of the flowpaths so as to maintain alaminar flow of said feedstock when the feedstock 12 is flowing throughthe flowpath at a predetermined operating pressure.

As shown in FIG. 4, each filter unit consists of a first backing plate16, a first barrier or membrane 13, a gasket 18, a second barrier ormembrane 13a and a second backing plate 16a. The backing plates 16 and16a have a peripheral sealing shoulder 19 which is operative when thefilter is assembled together to seal the periphery of the filter bagformed by membranes 13 and 13a. Alternatively, only one plate 16 couldbe provided with a shoulder 19 enlarged to engage the other plate 16a ina sealing manner.

Referring now to FIGS. 5 and 6, spacer support or backing plates 16,16a, 16b and 16b are arrange in a stack in spaced relationship to oneanother with pairs of compressible barriers or membranes 13, 13a; 13b,13c and 13d, 13e disposed between adjacent plates. Each barrier issupported by and spaced from each plate by a plurality of coinical studs29 formed on each surface of each plate with open volumes 20 formedtherebetween. Feedstock solution 12 is pumped between each pair ofbarriers which are compressed under the effects of the high transbarrieror transmembrane pressure which is created by the feedstock to form thinchannels 22, 23, and 24 splitting the feedstock solution 12 into aconcentrate or retentate 25 and a filtrate of permeate 11.

Referring to FIG. 7, each pair of opposed spacer support plates 27 and28 is provided with a plurality of conical studs 29 on the surfacethereof which serve to support a pair of membranes 30 and 31 disposedbetween the support plates. The barriers employed in this embodiment arecompressible and resilient, and, in this instance are multilayeredanistropic unltrafiltration membranes. Biofilters, filters or membranesprovided with a compressible and resilient backing may also be used. Theopen volumes 32 formed between the studs 29, when interconnected, form alow pressure filtrate or permeate pathway 33. Under conditions of noapplied pressure the barriers 30 and 31 would normally be in surface tosurface contact, with a flowpath thickness of zero--See FIG. 5. However,under the effects of the high transmembrane pressure which is created bythe flow of pressurized feedstock, each of the membranes 30 and 31 iscompressed elastically onto the conical studs 29 creating a thin channelor flowpath 34 of variable channel height "h" between the opposedsurfaces of the compressible membranes or barriers 30 and 31. Under theeffects of cross flow filtration or ultrafiltration, the feedstocksolution is split into a filtrate or permeate 33 which passes throughthe membrane and a concentrate or retentate 36.

As there will be a pressure drop along each flowpath channel 34 from thehigher pressure inlet side (left hand side of FIG. 7) to the lowerpressure outlet side (righthand side of FIG. 7), the thickness of theflowpaths will not be constant through the length from inlet to outlet.A slight tapering will occur because the higher inlet pressure willcompress the inlet zone of the membranes more than the lower outletpressure will compress the outlet zone of the membranes.

The distance between opposed spacer plates (typically about 250 microns)is given by "H", and the thickness of the membranes or filters is givenby "e" (See FIG. 5). In a dialysis system, where membranes are typicallyof about 40 micron thickness, the channel height between the membranesremains substantially constant and is predetermined as a consequence ofthe thickness of the membranes. In a conventional dialysis system H 2ewhereas for cross-flow ultrafiltration H 2e.

The preferred thickness of the flowpaths under steady state flow is thatwhich ensures that the elastic enlargement maintains laminar flow undernon-gel polarised conditions for prolong periods of time at high flux.As will be apparent from the foregoing description, it is the channelheight or flow path thickness whichf has alters the shear rate for agiven velocity of fluid. Thus, as the shear rate is inverselyproportional to flow path thickness, a reduction in flow path thicknesswill increase shear rate which will in turn increase flux.

Although variations in feedstocks may or will dictate that the flow paththickness is not a universally chosen parameter, it is preferred thatthe thickness does not exceed 80 microns or, in some cases 100 microns.In some instances the thickness range extends preferentially from 50 to100, microns, from 40 to 60 microns and from 10 to 25 microns.

The preferred compressible, high flux membranes used according to thepresent invention are the membranes disclosed in Australian Patentspecification No. 505,494. That specification discloses highly-permeableanistropic membranes with graduated porosity, comprising a mixture ofdepolymerised and polymeric material, and a plurality of adjacent layerswith each layer active as a molecular screen and having a precisemolecular weight cut-off, wherein the variation of molecular weightcut-off of the adjacent layers from the top to the bottom of themembrane is a continuous function.

In another form of the invention, the barriers could be constituted by acomposite of a first portion which is compressible across its transversedimension and a second portion which is substantially less compressibleacross its transverse, dimension, and wherein said limiting means isadapted to contact said first portion of said barrier when it restrictsthe extent of enlargement of the flowpath.

The gasket means 18 is preferably a compressible cellular or closed-cellfoam material, such as polyethylene or polypropylene, which underpressure is compressed from about 1 mm to about 15 microns. Undercompression, the cells in the foam material rupture forming a pluralityof open cell spaces in contact with the surface to be sealed. Each opencell structure acts, in effect, as a small decompression chamber, with alarge number of such chambers being present within a relatively smallspace, acting as an effective seal against loss of pressure in afiltration or ultrafiltration system operating under pressures (i.e.transmembrane pressure differentials) of about 100 KPa (or about 15p.s.i )--as opposed to pressure differentials of less than 10 KPa (orless than 1 p.s.i.) which exist in dialysis apparatus.

One means of providing for the supply of fluid to be treated underpressure to form a channel between a pair of filtration orultrafiltration media 13 with the latter adapted to provide afluid-tight seal with the adjacent plate 16 in the region surroundingthe inlet and the outlet openings as described above is to ultiliseradial fluid distribution discs or buttons 40 (See FIG. 1.) of the typedescribed in the Hagstrom et al U.S. Pat. Nos. 3,837,496 and 3,841,491between the pair of filtration or ultrafiltration media, coincidentalwith the inlet and outlet openings. However, the necessity of having aplurality of such distribution buttons 40 is detrimental to thecompactness of the system, and gives unnecessary flow restriction to thefeedstock and there is - under the high operative pressures which existin apparatus according to the present invention, the necessity toprovide an annular compressible sealing gasket (e.g. of polypropylenefoam material) on each side of, and concentric with, the button to forma seal under compression between the spacer support plate 16 and thefiltration or ultrafiltration media 13.

The active surface 42 of the spacer support plates 16, i.e. whereinpassageways are provided for the distribution and collection of thefiltrate or permeat 11, may be formed in many various ways according toprior art technique applicable to dialyzing apparatus technology, forexample using embossing and stamping techniques. In this regardreference is made to the surface structure 42 of the support plate 16disclosed in Miller et al U.S. Pat. No. 4,154,792--wherein the membranesupport surface comprises a large plurality of closely juxtaposedconical studs 29 or projections. In some embodiments of the apparatusaccording to the present invention the type of grooved or channeledmanifold structure of the support plate disclosed in the Riede U.S. Pat.No. 4,051,041 can be utilised, especially for the collection of filtrateor permeate from the active surface of the spacer support plate 16 intothe permeate outlet part 11 of the apparatus. See also the disclosure ofthe Alwall et al U.S. Pat. No. 3,411,630 relating to the surfaceconfiguration of the spacing members designed to provide a support forthe adjacent membrane and to provide a passageway for the dialysing ofpurifying liquid.

In a preferred form, the present invention provides the basis forfiltration apparatus having an energy input requirement as small asabout 50 and no more than 150 watts/square meter of membrane installed(c.f. an energy requirement of about 1 kW/square meter of membraneinstalled for classical prior art systems) which means that the presentinvention provides the basis for a significant saving in energyrequirements when compared to known prior art systems.

Another consequence of use of a preferred form of treatment systemaccording to the present invention is that the shear rate tends to beextremely high. As a result, the specific flux for a given effluent isincreased and it is thus possible to operate the filtration apparatusunder non-gel polarized conditions at high flux. This is of importancewhen it is desirable to maximise molecular selectivity and leads to aneasy cleaning of the barriers or membranes.

An advantage of a treatment system according to a preferred form of thepresent invention (apart from the energy saving) is that a very highsurface area of membrane of filter can be contained within a relativelysmall volume. Generally speaking, about 10 times more membrane or filterper unit of volume can be contained within a given area than is possiblewith classical prior art ultrafiltration or cross-flow filtrationequipment.

The use of deformation of the membrane itself to create the channel alsoresults in the very high stability of the filtration equipment accordingto a preferred form of the present invention. For example, if for anyreason the channel height or flowpath thickness h_(c) has a tendency todecrease, the surface area of the cross-section of the channeldecreases. This means that the pressure drop throughout the particularfiltration unit increases, which in turn means that the inlet pressureincreases resulting in a tendency for the channel height or flowpaththickness h_(c) to increase. Thus, there is an autostabilization orautoflush effect which facilitates cleaning of the unit.

In other words, if a channel 34 does become plugged by a cake 50 asshown in FIG. 10, the surface area of the channel will decrease and thepressure therein will rise. This causes the channel 34 to expand andopen (as shown in FIG. 11) to flush the cake 50 which has plugged thefiltration unit. This characteristic, known as the autostabilizationeffect is very important in respect of the self-cleaning ability of thefiltration system according to a preferred form of the presentinvention.

With membranes incorporated into the apparatus, the apparatus is adaptedfor cross-flow ultrafiltration. When the membrane is replaced by abiofilter or filter the equipment is suitable for cross-flow filtrationwith two separative effects:

(1) to remove the filtration cake constantly through high shear rate,and

(2) the tubular pinch effect.

In any given solution where the liquid medium and the particulate mattercontained therin are of different density, then it is possible to obtaina separation in two ways. If the particulate matter is heavier than theliquid, and if the feedstock solution (concentrate) is caused to flowupwards, i.e. in a substantially vertical direction, in the channel,then the particulate matter tends to concentrate in the centre of thechannel, with the result that it is possible to remove the permeatewithout plugging of the filter caused by the particulate matter.

On the contrary, when the particualte matter is lighter than the liquid,the feedstock concentrate is caused to flow in a downwards direction,again the particulate matter tends to agglomerate in the centre of thechannel, and again it is possible to remove the permeate withoutplugging of the filter caused by particulate matter.

Using the cartridge or filter apparatus shown in FIGS. 1 to 7 and ananisotropic nylon ultrafiltration membrane of the type whose propertiesand method of preparation are disclosed in Australian Pat. No. 505,494,tap water from Sydney, Australia was purified by ultrafiltration.

The tap water feedstock was recirculated through the cartridge over a 12hour period while the flux decline was monitored. Initially, theback-pressure was set at 88 kpa. After four hours, the pressure wasincreased to 100 kpa, the recommended minimum pressure for thisapplication. The cross-flow rate was 186 L/HR. and the temperature wasapproximately 30 degrees centrigrade. At an inlet pressure of 88 kpa,the stabilized flux was 64 L/SP.M.HR. The pressure drop across thecartridge was 20 kpa. The stabilized flux at 100 kpa was 74.3L/SQ.M.HR., and showed no decline over the last 8 hours of theexperiment. The flux versus pressure relationship for the cartridgeindicated that the experiment was carried out in a pre-gel polarizedcondition. The total dry solids content of 0.19 G/L for the feedstockand 0.08 G/L for the permeate gave a total solids rejection of 0.19 G/Lfor the feedstock and 0.08 G/L for the permeate gave a total solidsrejection of 60 O/O for this experiment.

Chemical analysis of the permeate indicated that it contained an averageof 2.5 PPM silicons, 12.9 PPM calcium, 5.0 PPM magnesium, and nomeasurable iron, manganese or copper. Hardness for this permeate wasalso determined as 5.3 MG./L as calcium carbonate equivalent, and thetotal dissolved solids were 15.2 UG/L.

The table below shows the power and energy requirements for theultrafiltration experiment based on membrane area and permeate volume:

Pump Type: Gear

Power Supply: Single Phase

Membrane Area (SQ.M.) 0.418

Power Consumption (KW): 0.098

Power per unit of membrane area (KW/SQ.M.) 0.21

Feed cross flow (L/HR.): 186

Membrane Flux (L/SQ.M.HR.): 74.3

Power per unit of permeate volume (MJ/CU.M.): 10.4

The above data shows the low power consumption per unit of purifiedwater, as well as the low power consumption per unit area of membrane.

A further aspect of the invention relates to the adaption of theapparatus for electrodialysis.

The particular configuration of the filtration modules of a preferredform of the present invention allows for the incorporation into the twoend plate manifolds of a stacked assembly of plates of two metallicplates as electrodes to establish an electric field. In this case, if,for example, separate anionic and cationic membranes were to be placedbetween the two metallic plates the filtration module would be adaptedto operate as an electrodialysis unit.

If one charged membrane and one neutral membrane were placed between thetwo plates the unit would be adapted to operate as a reverseelectrodialysis or transport depletion unit.

A second embodiment of the invention is shown in FIGS. 12 to 14. In thisembodiment, the apparatus includes a main body portion 60 having aninlet thereto 61 and an outlet therefrom 62. Within the main bodyportion 60 is a plurality of membrane envelopes 63. Each envelope 63includes a first membrane or barrier 64 and an overlying second membraneor barrier 65 which are held in spaced relation by grid 66. Theperiphery of the overlying barriers 64 65 is sealed together as shown inFIG. 14 except for one side thereof which projects into manifold 67 asshown in FIG. 13. Manifold 67 constitutes the transfer means for theapparatus and the space between each of the envelopes 63 is closed by asealing material such as aryldite as identified by numeral 68 in FIG.13.

When fluid is admitted to the body portion 60 under pressure thecontacting surfaces of opposed barriers 64 and 65 are separated in themanner described above in relation to the first embodiment of theinvention. The selected component passes through the barriers 64 65 intothe flow channel defined between the two barriers 64 65 by the grid 66and thence to the manifold 67 through the open ends 68 of the envelopes63 (see FIG. 13).

A further embodiment of the invention is shown in FIG. 15 wherein themembranes or barriers 70 and 71 have aligned and co-operatingprojections 74 and 75 which define flowpaths 76. In this way, theflowpath between the barriers 70 and 71 is divided into a plurality ofparallel flowpaths each of which is restricted in its elasticenlargement by the backing plate 72 and 73.

Yet another embodiment of the invention is shown in FIG. 16 whereinenvelopes 80 and 81 are spirally wound within each other. Each envelope80 and 81 is substantially similar to the envelope 63 shown in FIGS. 12to 14. Each envelope 80 and 81 has outlet means 82 and the assembly ofenvelopes is adapted to be positioned within a housing having an inletthereto and an outlet therefrom.

As indicated above, the invention is not limited to cross flowfiltration or retention. For example, the preferred form of theinvention described in relation to FIGS. 1 to 7 could be used tointroduce a preferred species into the feedstock by entry through thetransfer port and the barrier(s) in the reverse direction to thatdescribed above.

One such application concerns the introduction of oxygen (the preferredspecie) into blood (the feedstock). In this instance, the barriers areso chosen that the oxygen may readily flow thereacross but allcomponents of the blood are retained in the flowpaths.

What is claimed is:
 1. Cross-flow filtration apparatus for removing apreselected component from a fluid feedstock, which comprises:a firstplate member having a plurality of closely spaced fixed projections,said projections having tips that are in a first plane; a second platemember having a plurality of closely spaced fixed projections, saidprojections of said second plate member having tips that are in a secondplane; means supporting said first and second plate members in a fixed,spaced relationship to each other with the projections of said firstplate member facing and in a fixed relationship to the projections ofthe second plate member; first and second compressible, resilientmembranes of sheet material through which the preselected component canpass, each of said membranes having a first surface and an oppositesurface; said first membrane having its first surface thereof in contactwith the first surface of said second membrane when there is no flow offeedstock; said first membrane having its opposite surface in contactwith the projections of said first plate member and said second membranehaving its opposite surface in contact with the projections of saidsecond plate member; said supporting means for said first and secondplate members maintaining said first and second planes at a fixeddistance from each other that is no greater than the non-compressedcombined thickness of said first and second membranes located betweensaid first and second planes; inlet means for introducing pressurizedfeedstock into the apparatus and between the first and second membranesto force the compressible, resilient menbranes themselves to becomecompressed against the fixed projections of said first and second platemembers by the pressurized feedstock passing through the apparatus, andthus separate the membranes from each other as a result of the distancebetween the first and opposite surfaces of each membrane becomingsmaller due to compression, to form a feedstock flow path between themembranes.
 2. The apparatus of claim 1 wherein the membranes haveperipheries and further including means sealing the peripheries of saidmembranes to render the flow path leakproof.
 3. The apparatus of claim2, wherein the means sealing the peripheries of said membranes comprisecompressed gasketing disposed between the peripheries of the membranes.4. The apparatus of claim 3 wherein the gasketing is provided by acompressible cellular or closed-cell foam material which, underpressure, is compressed, so that the cells in the foam material ruptureforming a plurality of open cell spaces in contact with a surface to besealed.
 5. The apparatus of claim 1 wherein each membrane iscompressible across its entire transverse dimension.
 6. The apparatus ofclaim 1 wherein each membrane comprises a composite of a first portionwhich is compressible across its transverse dimension and a secondportion which is substantially less compressible across its transversedimension, and wherein said projections contact said first portion ofeach membrane.
 7. The apparatus of claim 1 comprising a stackedarrangment of membranes and plate members and wherein projections areformed on each side of each plate member.
 8. The apparatus of claim 1wherein the spacing of the projections and the compressibility of themembranes are such that when the feedstock in the flow path exceeds itsprescribed operating pressure because of a blockage in the flow path,the membranes further compress to expand the flow path furthertemporarily but repeatedly at the location of the blockage to permit theblockage to proceed through said flow path until the blockage is removedfrom the flow path.